1. Field of the Invention
This invention pertains to electrical resistors generally and specifically to resistor configurations that are, on occasion, subjected to large surges of electrical energy.
2. Description of the Related Art
Electrical resistors may be formed using a variety of processes such as screen printing, vapor deposition, compaction, lamination, and immersion plating. Film type resistors, herein considered to be resistors that have a thin film of resistive material deposited upon a non-conductive substrate, are most commonly formed from vapor deposition and screen printing techniques. Other processes to form resistors, such as winding and compaction, result in carbon pile, wire-wound, and other resistors.
In electrical applications electrical transients occasionally occur upon failure of components, applied voltage surges such as improper connection of a power source, or induced signals from neighboring equipment. Transients of sufficient magnitude can cause failure of resistors, including resistors that form a part of circuitry specifically designed to protect other circuitry from the surge. Transients of large magnitude often adversely affect film type resistors.
A resistor that has failed because of an electrical surge usually has tell-tale signs. Electrically generated thermal energy usually concentrates about one or several localized regions. The localized heating may cause separation of the resistive material from the substrate, separation of the resistive material, separation of the substrate material, drift in the resistance component value, or a melting or fusing of materials. The prior art in U.S. Pat. No. 2,910,664 to Lanning, U.S. Pat. No. 3,468,011 to Curtis, U.S. Pat. No. 4,245,210 to Landry et al., and U.S. Pat. No. 4,647,900 to Schelhorn et al. discuss various methods for reducing the ill effects of surges.
Lanning discloses the formation of a particular termination geometry that extends transversely to a resistor element to prevent current crowding from occurring in the resistor material close to the termination. In this disclosure, any design changes influence the performance of a resistor only at the terminations. While in some applications this may be invaluable, there are other applications or resistor configurations which require control of current crowding or thermal "hot spots" within the body of the resistor. The Lanning disclosure also lacks features to adjust for variations in thickness or for voids at the interface between resistor and termination, both which are common in screen printed resistors.
Curtis discloses the separation of a single resistor body into several discrete elements which then divide the current flow. The Curtis design limits current crowding with resistor paths having length very nearly equal to diagonal measure. Additionally, current then divides between many locations to reduce the concentration of heating. However, the Curtis disclosure also requires formation of fine lines as opposed to the formation of a single large block. The minimum size of resistive material that may be patterned without complete loss of conductivity due to the formation of voids, micro-cracks or other defects limits applicability of the Curtis disclosure. Further, while the Curtis disclosure does provide for better thermal distribution than the prior art illustrated by Curtis, there are still many discrete regions (as opposed to one) that may be elevated to harmful or destructive temperatures during a transient surge. In effect, this design does not eliminate the electro-thermal heating at the terminations, but rather divides one "hot spot" into several spots.
Landry et al. disclose the use of multiple layers of high resistance material to reduce current crowding resulting from voids, non-homogeneity, and geometry irregularities such as surface roughness and thickness of deposited films. However, the Landry et al. resistor requires completely compatible and migration-free materials to prevent resistance drift with environmental cycling. Further, in screen printing applications, the use of multiple layers implies a very thick resistive film that uses excessive material and may be more likely to form cracks during firing and operation. Additionally, Schelhorn has identified the migration of conductive during multi-step firing as another concern for the Landry et al. design.
Schelhorn et al. disclose the formation of a first relatively conductive resistor material that extends between electrical terminations and a second resistor material of relatively greater resistance applied over the first resistor material. This combination is said to offer many of the advantages of the Landry et al. disclosure without the expense and loss of yield associated with multiple firing processes. Both materials of the Schelhorn et al. disclosure must be present virtually from one terminal to another. This co-extensive application may carry a large materials expense, particularly in those situations that require precious metal materials and sizable resistors. Additionally, the Schelhorn resistor may experience greater resistance drift with environmental cycling if the two resistive materials are not completely compatible and free from migration. In summary, while migration during firing may be reduced in comparison with the Landry et al. disclosure, the large material usage associated with the second high resistance layer and the drawbacks inherent to both the Landry and Schelhorn design makes these approaches less than ideal.
In summary, the prior art is limited to particular geometries or configurations that are not applicable to the field of electrical resistors in general.